In recent years the use of high strength-to-weight ratio fiber reinforced resin composites has continuously expanded, particularly in weight sensitive products such as aircraft and space vehicles. In the past, fiber reinforced resin composites used in such products were usually created by "laying-up" a plurality of unidirectional plies using tapes having a prescribed fiber areal weight preimpregnated with resin. (Areal weight is weight per unit area, i.e., grams per square meter.) The ply direction and number of plies included in such layups is based on the ultimate use of the resultant composite. In most instances a main ply direction (commonly referred to as the 0.degree. ply direction) is first chosen based on some criteria, such as the direction of the major force to be applied to the composite. After the main or 0.degree. ply direction is chosen, the direction of other plies are chosen based on the forces and loads to be applied to the resulting composite. The most common directions are: orthogonal to the main direction (commonly referred to as the 90.degree. direction); and equally intermediate the main and orthogonal directions (commonly referred to as the + 45.degree. and -45.degree. or + and - directions). In addition to determining ply direction, the percent of fiber orientation in each direction was determined based on expected load force. For example, a prescribed fiber orientation for an aircraft wing might be: 6% in the 0.degree. direction; 44% in the +45.degree. direction; 44% in the -45.degree. direction; and 6% in the 90.degree. direction. After the foregoing choices have been made, the layup is created using tape of a chosen grade having a prescribed number of filaments per tow.
The major disadvantage of the foregoing process is that it requires that undesirable design compromises be made. Specifically, the tapes used to make prior layups have a prescribed fiber areal weight and, thus, a prescribed thickness. In many instances, the fiber areal weight is such that more than that required to meet the design criteria of a particular ply direction is provided by a particular tape. While this result can be limited in some situations by changing to a grade of tape having a lower fiber areal weight, this approach does not fully solve the problem since tape grades are fixed. Thus, if only one ply in a particular fiber orientation direction is required often it must contain more than the design number of fibers since it is unusual for design criteria and tape sizes to match in the area of fiber areal weight. A similar situation exists when tape fiber areal weight is not a whole number multiple of the design fiber areal weight in a particular fiber direction. In such instances, in the past, more fiber tape than necessary has been used since a whole ply was included to achieve the strength required of the last (partial) ply. Hence, the end result of past techniques for creating fiber reinforced resin composites is composites that are thicker and, thus, heavier, than necessary.
Another disadvantage, particularly in instances where only a single ply is needed in a particular direction, is fiber concentration. Fiber concentration in a single layer, as opposed to spreading fibers through several layers, has the disadvantage of creating a higher probability shear plane, i.e., a plane along which shear is more likely to occur.